A plurality of tendon tensioning apparatus are available such as strand chucks, strand vices, and other tendon-engaging devices used with a cooperating hydraulic jack or other means by which the tendon is tensioned by pulling on it. In such apparatus, the tendon-engaging device serves to maintain the tendon tension when the pulling force applied by the hydraulic jack is removed. While such apparatus does afford the means by which torsion-free axial tensile stress may be induced in a tendon, the apparatus and method are useful only when a device for pulling on the tendon is available and also only when the space required for use of such a device is available. This invention is directly primarily to those conditions in which it is desirable or necessary to induce torsion-free axial tensile stress in a tendon or other linear member and in which it is also desirable or necessary to do so without the aid of tendon pulling means, but with only cooperating torque wrenches.
A tendon-engaging device like the one disclosed by Drouillard, U.S. Pat. No. 3,478,396, for example, when used with a torque wrench but without a tendon pulling device, can be used to induce at least some tension in a tendon, but only to the degree to which the tendon by itself is able to resist the torque required to rotate the nut. Newton's Laws and other well known principles of mechanics will show that for each measure of torque applied to the nut to cause it to rotate, there will be a corresponding measure of torque or proportional magnitude induced in the tendon. The result is, therefore, not only axial tensile stress, but also co-existing torsion stress. The amount of torque which can be applied to cause the nut to rotate is therefore limited by the amount of torque which the tendon can tolerate.
The avoiding of also inducing torsion stress in a tendon as it is tensioned is desirable and frequently necessary. In applications involving tendons which are to be highly stressed in tension, co-existing torsion stress is known to limit the magnitude of the tension stress which the tendon can endure without failure and to adversely affect the reliability of the tendon. Some tendon forms, stranded wire or cable for example, may unwind when subjected to torsion stress.
Prior art, Drouillard for example, discloses the use of threads on a sleeve member and the use of the threads for engagement by a threaded coupling device to which a tendon pulling force is applied. Such threaded engagements are not effective in theory or in practice in the avoiding of tendon torsion stress as the nut is rotated because, as is well known in the art, such engagements are provided for the very purpose of accomodating relative rotation of the two parts so engaged and not for the purpose of restraining relative rotation.
The foregoing discussion of representative prior art is included here to illustrate its significant limitations and to call attention to the important and distinctive characteristics of this invention. As can be seen, a need exists for an apparatus and a method which makes it possible in theory and practice to induce torsion-free axial tensile stress in a tendon using only torque wrenches in the process. The present invention is directed to that need.